A valve regulated lead–acid (VRLA) battery, commonly known as a sealed lead–acid (SLA) battery, is a type of lead–acid battery characterized by a limited amount of electrolyte ("starved" electrolyte) absorbed in a plate separator or formed into a gel; proportioning of the negative and positive plates so that oxygen recombination is facilitated within the cell; and the presence of a relief valve that retains the battery contents independent of the position of the cells. There are two primary types of VRLA batteries, absorbent glass mat (AGM) and gel cell (gel battery). The lead–acid gel batteries contain a mixture of sulfuric acid and finely divided silica. This mixture forms a thick paste or gel, thereby giving the batteries the name - Gel Cell. Gel batteries can be made with either flat or tubular positive plates. AGM batteries feature fiberglass mesh or an ultra thin glass mat (called AGM separator) between the battery plates which serves to contain the electrolyte and separate the plates. This additional layer absorbs the electrolyte and makes the battery non-spillable. Both types of VRLA batteries offer advantages and disadvantages compared to flooded vented lead–acid (VLA) batteries or each other. The gel provides a better means of heat conduction from the plates to the cell walls than in AGM batteries. Since the heat produced on overcharge is lost more efficiently, they are suitable for deep cycle applications. AGM batteries, on the other hand, offer fastest charging rate, offer long service life, and are maintenance-free. Due to their construction, the gel cell and AGM types of VRLA can be mounted in any orientation, and do not require constant maintenance. The term "maintenance free" is a misnomer as VRLA batteries still require cleaning and regular functional testing. They are widely used in large portable electrical devices, off-grid power systems and similar roles, where large amounts of storage are needed at a lower cost than other low maintenance technologies like lithium ion. History The first lead–acid gel battery was invented by Elektrotechnische Fabrik Sonneberg in 1934. The modern gel or VRLA battery was invented by Otto Jache of Sonnenschein in 1957. The first AGM cell was the Cyclon, patented by Gates Rubber Corporation in 1972 and now produced by EnerSys. The cyclon is a spiral wound cell with thin lead foil electrodes. A number of manufacturers seized on the technology to implement it in cells with conventional flat plates. In the